Belt tensioners are generally well known devices that have been used previously in many belt-drive systems. It is conventional practice to use a tensioner to apply a constant belt-tensioning force, which compensates for increases in belt length due to wear and other factors. A common type of conventional belt tensioner has a fixed structure and a pivoted structure eccentrically mounted on the fixed structure by means of a pivot assembly, and the pivoted structure has a belt-engaging pulley rotationally mounted on it. A coil spring surrounds the pivot assembly and has its ends connected between the fixed and pivoted structures so as to bias the pivot structure in a belt take-up direction. As the pivoted structure moves from a position of minimum belt take-up to a position of maximum belt take-up, the spring biasing force decreases. Despite this varying spring force over the range of tensioner movement, substantially constant belt tension is maintained by the tensioner. U.S. Pat. No. 4,473,362, for example, illustrates these basic principles.
Various techniques are currently used to properly install timing belt tensioners on engines. One of the most commonly used techniques is to construct the tensioner with an eccentric adjusting member that forms part of the fixed structure; the eccentric adjusting member is rotated around the tensioner mounting bolt and thus moves the tensioner away from the belt (to allow the belt to be routed into the drive system) or towards the belt (to apply tension in the drive system). A typical installation procedure when using the current standard design includes mounting the tensioner on the engine with the eccentric member in the extreme position away from the belt, routing the belt into the drive system, rotating the eccentric member towards the belt until the tensioner reaches the nominal operating position, and locking the tensioner with the mounting bolt.
Because the eccentric adjusting member is located within the periphery of the tensioner pivot, its size is limited and the maximum linear stroke of the tensioner into and out of the drive system (approximately equal to twice the eccentricity of the eccentric member) may be insufficient to allow for proper installation of the belt. Additionally, recent trends to increase the number of components in the timing drive system and to increase the tolerance range on belt dimensions, as well as requirements from engine OEM's that belts from more than one supplier (i.e., belts with different tolerances) can be used on the same timing drive system, make it very difficult for a tensioner equipped with a regular eccentric member to have enough installation travel to accommodate all situations.
In order to address situations where the installation stroke provided by the above design does not sufficiently move the tensioner away from the belt to allow the belt to be routed into the drive system, an improved design was developed to increase the installation stroke. In particular, in such improved design tensioners, an installation pin locks the pivoted structure to a fixed portion of the tensioner assembly at an extreme position away from the belt (known as the load stop position). The pin is inserted through the pivoted structure into a fixed component (e.g., base plate, shaft, front plate, etc.) and counters the action of the spring biasing member (tending to force the pivoted structure toward the belt). The installation procedure using the enhanced design includes mounting the tensioner on the engine with the eccentric member in the extreme position away from the belt, routing the belt into the drive system, releasing the pin, rotating the eccentric member towards the belt until the tensioner reaches the nominal operating position, and locking the tensioner in place with the mounting bolt. This improved design increases the amount of space available to install the belt by holding the pivot arm as far away from the belt as possible.
A disadvantage of such “enhanced” design, however, is that the pin is loaded by the spring (because the pin is required to hold the pivot arm at the load stop position, away from the belt). As a result, it can require significant force to remove the pin from the arm and the fixed component, which makes pin removal (and hence the tensioner installation procedure) difficult. Furthermore, when the pin is removed and the pivoted structure is “released” to pivot freely, the pivoted structure may pivot all the way into contact with the tensioner free arm stop with significant impact force. Such impact force can damage the internal components of the tensioner, particularly if the tensioner uses a high torque spring.